An embodiment of the invention relates to antiscattering grids used in X-ray imaging. As illustrated in FIG. 1, a radiology imaging apparatus conventionally comprise means for providing a source of radiation 1, such as an X-ray source, and means for detecting emitted radiation 2, such as an image receiver 2. The object 3 for which an image is to be produced is located between the source and the receiver. A beam emitted by the source 1 passes through the object before reaching the detector 2. The beam is partly absorbed by the internal structure of the object 3 such that the intensity of the beam received by the detector is attenuated. The global attenuation of the beam after passing through the object 3 is directly related to the distribution of absorption in the object 3.
The image receiver 2 comprises an opto-electronic detector or a reinforcing film/screen pair sensitive to the radiation intensity. Consequently, the image generated by the receiver corresponds (in principle) to the distribution of global attenuations of rays, due to passing through internal structures in the object.
Part of the radiation 4 emitted by the source 1 is absorbed by the internal structure of the object 3, and the remainder is either transmitted or scattered. In the remainder, the transmitted radiation 5 is referred to as “primary radiation” (or direct radiation) and the scattered radiation 6 is referred to as “secondary radiation”. The presence of secondary radiation 6 degrades the contrast of the image obtained and reduces the signal/noise ratio. This is particularly of concern when it is required to display details of the object 3.
A solution to this problem comprises inserting an “antiscattering” grid 7 between the object 3 to be X-rayed and the image receiver 2. This grid is positioned in a plane parallel to the plane comprising the image receiver 2. The plane of the grid will be called the grid positioning plane in the remainder of this document.
As illustrated in FIG. 1, an antiscattering grid 7 comprises a periodic arrangement of parallel plates 8 with height h maintained within the inter-plate members 9. The plates 8 are composed of a dense material strongly absorbent of X-rays, and the inter-plate members 9 are filled with a material more transparent to X-rays. The plates 8 are at a constant pitch or period. This pitch corresponds to the spacing 10 measured center-to-center (the center of a plate corresponding to its center of symmetry) between two plates 8, or the spacing 11 measured edge-to-edge between two plates 8. The concept of aperture “O” is also defined, corresponding to the distance 12 between faces facing of two successive plates 8, in other words the width of inter-plate members 9. The antiscattering grids 7 considerably improved the contrast of the images obtained. These grids 7 allow primary radiation 5 to pass through, and absorb secondary radiation 6.
An antiscattering grid is characterised particularly by three parameters, namely a primary radiation transmission ratio Tp, a secondary radiation transmission ratio Ts and application limits. The primary radiation transmission ratio Tp is related to the fact that primary rays 5 are attenuated by the plates 8 due to the non-zero width of these plates 8 and absorption of the inter-plate members. The secondary radiation transmission ratio Ts is related to the fact that some secondary rays pass through the grid at the inter-plate members 9. The application limits define a range of distances from the source at which the grid can be placed while maintaining an acceptable attenuation level on the edges (for example as defined in standard IEC 60627).
In order to obtain a good quality grid, it will be necessary to: maximize the primary radiation transmission ratio Tp that contains useful information; minimize the secondary radiation transmission ratio Ts that reduces the image contrast; and maximize application limits that define the range of grid/source distances at which the grid can be placed. The secondary radiation transmission Ts depends on a ratio R called the “grid ratio”. This grid ratio R is equal to the quotient of the plate height h divided by the aperture O:
  R  =            h      O        .  
Prior art solutions to improve the quality of antiscattering grids are based particularly on minimizing transmission of secondary radiation Ts. One solution comprises increasing the grid ratio R by increasing the height h of the plates 8 while maintaining the same aperture O. However, this solution has the following disadvantages: transmission of primary rays becomes more sensitive to alignment defects of plates 8 with the X-ray source (as the grid ratio increases, the transmission of primary rays becomes more sensitive to defocusing of plates with respect to the source); application limits are smaller; the primary radiation transmission ratio Tp is reduced; the increase in the height h of the plates 8 induces an increase in the height of the inter-plate members; and consequently, the length of the imperfectly transparent material that the X-rays have to pass through is greater, inducing greater attenuation of X-rays.
Another solution comprises reducing the aperture O while keeping the same height h for the plates 8. However, this solution has the following disadvantages: transmission of primary rays becomes more sensitive to alignment defects of plates 8 with the X-ray source; application limits are smaller; and the primary radiation transmission ratio Tp is reduced; the reduction in the aperture for the same plate width induces an increase in the relative surface area occupied by the edges of the plates, and therefore a greater attenuation of X-rays. Thus, even if the increase in the grid ratio R can help to improve elimination of secondary radiation, it also degrades transmission of the primary radiation. This attenuation of the primary radiation causes an increase in the X-ray dose emitted to the patient to obtain a useable image, which is not desirable.